The present invention relates to controlled mechanisms where at least one mode of control is exercised by a piezo ceramic device,
More particularly, the present invention relates to an over-centre mechanism where a piezo-ceramic device is used to control the mechanism whereby to collapse the mechanism when locked.
The use of over-centre mechanisms is known as a means of applying high clamping forces to electrical contacts, valve faces and frictional holding devices. The geometry of over-centre clamping mechanisms is also known, wherein the linkages in the mechanism must pass from one mode in which they are free to rotate, past the centreline of the plane of action and into a mode where they are urged to rotate by the reaction in the plane of action but are stopped by some mechanical means.
In order to readily collapse the mechanism, the means of stopping rotation can be withdrawn or otherwise collapsed, allowing spring forces to push the system open. Alternatively, the mechanism can be pulled back to rotate in the direction in which it was originally set. These actions are readily achieved with solenoids or fluidic pistons. The magnitude of the force necessary to cause the mechanism to collapse is generally lower than that required to set the system.
Electro-strictive piezo ceramic actuators are also known for the operation of mechanical devices, having the benefit of high speed, compact size and low power. Piezo devices are made in two basic forms. One type of device relies upon the basic change in dimension of the material. The change is measured in low parts per thousand, so devices of this type are generally suited only for micro positioning or ultrasonic transduction.
The other type of device acts like a bimetallic strip where the ceramic is bonded to a thin strip of metal to create a bending action. Although these devices have more movement it is still generally less than 1 mm and the output force is very low. Both types of devices are available in multi-layer forms which increase motion and reduce operating voltage.
In order for a piezo actuator to be used for the release of an over-centre mechanism suited to mass production it is necessary to combine higher force with higher displacement.
It is an object of the present invention to provide an over-centre mechanism in which a locked condition of the mechanism is released by an electrically actuable pivot guide releasing a pivot of a link of the mechanism by moving the pivot guide in a direction out of the plane of motion of the linkage, thereby allowing free motion of the pivot on the plane of motion and collapsing the mechanism.
In order to meet the above object according to the present invention there is provided an over-centre clamping mechanism comprising an actuating lever member arranged to rotate about a fixed pivot, a link arm member rotatably attached to said actuating lever member by a first movable pivot, and an actuable pivot guide means arranged to constrain the movement of a second movable pivot provided on said link arm member within a plane of action, wherein said actuable pivot guide means is electrically actuable to move out of said plane of action to release said second movable pivot whereby to collapse the clamping mechanism when said clamping mechanism is set.
The actuable pivot guide means may further comprise a piezo-ceramic bender actuator arranged to bend in a direction away from the plane of motion of the mechanism in order to allow the guide pivot to be released.
Furthermore, in a first mode of operation the guide means may constrain the guided pivot to move along a line on the plane of motion of the mechanism, and in a second mode of operation when the guide means have moved out of the plane of motion, the guided pivot may be free to move on the plane of motion.
The over-centre mechanism may further comprise a carrier arm arranged to rotate about a second fixed pivot point in response to the action of an action point of the link arm member there against, in order to force a free end of the carrier arm against a fixed portion in order to lock the mechanism.
The carrier arm and the fixed portion may each be provided with electrical contacts thereon, arranged to contact when the mechanism is locked.
Instead of being of single piece construction, the link arm member may comprise two link arms, one connected between the first movable pivot on the actuating lever member and the guided pivot, and the other connected between the guided pivot and a third pivot provided on the carrier arm member.